The present invention concerns a method and a device for the geometric transcoding of a compressed digital signal. It also concerns a coding method and device on the one hand, and a decoding method and device on the other hand, associated with the geometric transcoding method and device.
The technical field of the invention is that of methods for manipulating compressed digital data.
Geometric transcoding is a transformation of one compressed digital data file into another compressed digital data file which, once decoded, represents a digital signal which has undergone a geometric transformation with respect to the initial coded digital signal in the initial compressed data file.
The geometric transformation of a mono- or multi-dimensional digital signal consists of changing the order of the samples of digital signal in one or more directions of the digital signal.
In practice, in the case for example of an image, axial or central symmetry can be produced, or a rotation of the image by manipulating the digital signal representing the image.
It is particularly advantageous to be able to carry out this manipulation and transformation of a coded digital signal without having to compress and then once again compress the file containing the coded digital signal, in order notably to reduce the number of calculations and the memory space necessary for a decompression of the compressed digital data file.
Techniques are known for manipulating digital data in a compressed form, which have been coded by a coding method based on a discrete cosine transform (DCT). Such techniques are described for example in the article xe2x80x9cA new family of algorithms for manipulating compressed imagesxe2x80x9d by Brian C. Smith, Lawrence A. Rowe, IEEE Transactions on Computer Graphics and Applications, September, 1993, and in U.S. Pat. No. 5,751,865 (Felice A. Micco, Martin E. Banton).
However, the techniques of compressing a digital signal based on a discrete cosine transform are less efficient in terms of compression than the compression techniques based on a spectral breakdown of the digital signal into frequency sub-bands.
The present information aims to remedy the drawbacks of the prior art by proposing a method and device for transcoding a compressed data file by a compression technique using a spectral breakdown of the digital signal.
To this end, the invention concerns a method for the geometric transcoding of a compressed data file containing a digital signal of dimension N coded by a coding method including at least one step of spectral breakdown into frequency sub-bands of the digital signal.
According to the invention, this transcoding method is characterised in that it includes the following steps:
extraction of symbols associated with the coefficients of the frequency sub-bands in each direction of the digital signal;
application of a geometric transformation to said symbols;
updating of N indicator or indicators representing a normal or reversed order of the symbols respectively in N direction or directions of the digital signal; and
reconstitution of the coded digital signal by reversal of the extraction step
Correlatively, the invention proposes a device for the geometric transcoding of a compressed data file containing a digital signal of dimension N coded by a coding method including at least one step of spectral breakdown into frequency sub-bands of the digital signal, characterised in that it has:
means of extracting symbols associated with the coefficients of the frequency sub-bands in each direction of the digital signal;
means of applying a geometric transformation to said symbols;
means of updating N indicator or indicators representing a normal or reversed order of the symbols respectively in N direction or directions of the digital signal; and
means of reconstituting the coded digital signal by reversal of the extraction step.
The geometric transcoding method and device make it possible to effect a geometric transformation of a coded digital signal on the basis of a spectral breakdown without completely decompressing the digital signal since the precaution is taken of modifying an indicator intended to give information about the reversal or not of the frequency sub-band symbols, in each direction of the digital signal. This indicator makes it possible to adapt the subsequent decoding method, notably the spectral reconstruction of the digital signal as a function of the value of this indicator.
In addition, this transcoding method avoids making modifications to the value of the symbols associated with the coefficients of the frequency sub-bands of the digital signal, without any increase in the calculation quantity necessary at the time of decoding.
According to one advantageous characteristic of the invention, the extraction step includes a reading of the coded digital signal, the signals being entropic codes associated respectively with the coefficients of the frequency sub-bands obtained by spectral breakdown.
This embodiment is particularly advantageous since it requires a minimal decompression of the compressed data file, the geometric transformation being applied directly to the entropic codes contained in the compressed data file, referred to as prefix codes. It is, however, well suited only when the coding of the digital signal uses an entropic coding, of the Huffman coding type, which associates an entropic coding with each coefficient of the signal of the frequency sub-bands.
According to a preferred version of the invention, which minimises the decompression of the file and is applied for a large number of conventional coding methods, using a scalar quantization of the coefficients of the signal of the sub-bands, the extraction step includes an entropic decoding of the coded digital signal, the symbols being quantization symbols associated respectively with the coefficients of the frequency sub-bands obtained by spectral breakdown.
Alternatively, according to another preferred version, which avoids the complete decompression of the file, and notably the spectral recomposition of the digital signal, the extraction step includes a dequantization of the coded digital signal, the symbols being the dequantized coefficients of the frequency sub-bands obtained by spectral breakdown.
This embodiment is particularly well suited when the coding method uses a vector quantization of the digital signal broken down into frequency sub-bands.
According to a preferred version of the invention, the indicator is a supplementary bit recorded in the compressed data file, having an initial value representing a normal order of the coefficients of the frequency sub-bands in a direction associated with said indicator.
Such an indicator is particularly advantageous in terms of memory space used in the compressed data file. It is also very easily updated from its initial value, fixed for example at the time of coding of the digital signal which has not undergone any geometric transformation, for example by alternation of the values 0 and 1 representing respectively a normal or reversed order of the symbols, for each direction of the digital signal.
In a preferred implementation of the invention, which applies in particular to geometric manipulations of the images, for example, for printing them, the digital signal then being of dimension 2, the geometric transformation applied is a transformation by axial or central symmetry of said symbols, a transformation by rotation through a multiple of 90xc2x0 or a combination of said transformations.
In this same preferred embodiment of the invention, the transcoding method also comprises a step of transposition of a frequency sub-band having coefficients of low frequency in a first direction of the digital signal and high frequency in a second direction of the digital signal with a frequency sub-band with the same resolution level in the spectral breakdown, having coefficients of high frequency in said first direction and low frequency in said second direction, when the geometric transformation applied comprises a rotation through 90xc2x0 or 270xc2x0.
This characteristic of the transcoding method makes it possible to take into account the changes in direction of the symbols during the rotations through 90xc2x0 or 270xc2x0 of an image for example, for a correct application of the method of decoding the digital signal.
According to another preferred version of the invention, the compressed data file containing several digital signals sequenced in a pre-determined order, the transcoding method also includes a step of sequencing the digital signals according to the geometric transformation applied.
The transcoding method thus applies particularly well to a digital signal divided into digital subsignals before being coded, the subsignals being stored in a pre-determined order in the compressed data file.
The invention also concerns a method of decoding a compressed data file containing a digital signal of dimension N coded by a coding method including at least one step of spectral breakdown into frequency sub-bands of the digital signal, characterised in that it comprises, in each direction of the digital signal, the following steps:
reading an indicator representing a normal or reversed state of coefficients of the frequency sub-bands in said direction of the digital signal;
calculating the parity of the digital signal in said direction;
transforming original spectral recomposition filters in said direction as a function of the parity of the digital signal and the value of the indicator; and
spectral recomposition of the digital signal by means of transformed recomposition filters.
Correlatively, the invention also concerns a device for decoding a compressed data file containing a digital signal of dimension N coded by a coding method including at least one step of spectral breakdown into frequency sub-bands of the digital signal, characterised in that it has:
means of reading an indicator representing a normal or reversed state of coefficients of the frequency sub-bands in a direction of the digital signal;
means of calculating the parity of the digital signal in said direction;
means of transforming original spectral recomposition filters in said direction as a function of the parity of the digital signal and the value of the indicator; and
means of spectral recomposition of the digital signal by means of transformed recomposition filters.
This decoding method and device make it possible to decode a digital signal taking account of any reversal of the coefficients of the signal of the frequency sub-bands for the spectral reconstruction of the digital signal.
The transformation of the reconstruction filters can be effected in a relatively simple fashion, according to the parity of the digital signal and the reversal or not of the coefficient of the signal.
According to an advantageous version of the invention, which uses a relatively simple transformation of the filters, the time of the transformation of the original spectral recomposition filters, these filters are made symetrical and/or offset by an index.
According to a preferred version of the invention, particularly well adapted when the digital signal coding method comprises, in each direction of he signal, a processing of the start and a processing of the end of the digital signal, the decoding method according to the invention also includes the following steps:
calculating the parity of the spectral recomposition filters;
choosing the processing to be applied to said digital signal as a function of the parity of the spectral recomposition filters, the parity of the signal and the value of the indicator; and
applying said chosen processing to the digital signal before the spectral recomposition step.
The decoding method thus makes it possible to take into account the edge problems which generally present themselves at the end of a finite monodirectional digital signal. It makes it possible to reverse the processing to be applied to the ends at the time of decoding if the coefficients of the sub-band signal have been reversed.
In an advantageous version of the invention, particularly simple to implement, the digital signal start and end processings are symetrical extensions of the digital signal.
According to a preferred version of the invention, the decoding method also includes a prior step of dividing the compressed data file into several digital signals sequenced in a pre-determined order, the size of said signals being determined according to the value of the indicators.
The decoding method thus takes account also of the change in the order of signals made at the time of transcoding of the signals, so as to re-divide the transcoded digital signal into sub-signals corresponding in size to the sub-signals divided at the time of coding of the digital signal.
The invention also concerns a method of coding a digital signal of dimension N adapted to be transformed geometrically by a transcoding method according to the invention, characterised in that it comprises the following steps;
spectral breakdown into frequency sub-bands of the digital signal; and
entry into a compressed file comprising the coded digital signal of N indicator or indicators associated respectively with N direction or directions of the digital signal, in the form of a supplementary bit having an initial value representing a normal order of the coefficients of the frequency sub-bands in a direction associated with said indicator.
Correlatively, the invention concerns a device for coding a digital signal of dimension N adapted to be transformed geometrically by a transcoding method according to the invention, characterised in that it has:
means of spectral breakdown into frequency sub-bands of the digital signal; and
means of entry into a compressed file containing the coded digital signal of N indicator or indicators associated respectively with N direction or directions of the digital signal, in the form of a supplementary bit having an initial value representing a normal order of the coefficients of the frequency sub-bands in a direction associated with said indicator.
This coding method and device has the advantage of associating with the digital signal, as from coding, an indicator representating a normal, non-transformed state of the digital signal.
By associating an indicator with each direction of the digital signal, it is then possible, at the time of transcoding, to modify these indicators in order to take account of the transformations of the digital signal, in a compressed form, in all the directions of the space containing the digital signal.
The invention also relates to a digital signal processing apparatus having means adapted to implement the transcoding method, the decoding method or the coding method, or again having a transcoding device, a de-coding device or a coding device as disclosed above.
The transcoding, decoding and coding methods according to the invention are particularly well adapted to be used in a digital photographic apparatus, a computer or a photocopier.
Correlatively, the invention also concerns a computer, a photocopier or a digital photographic apparatus having a transcoding device, a decoding device or a coding device according to the invention.
The transcoding and decoding methods are also particularly well adapted to be used in a digital printer.
Correlatively, the invention also concerns a digital printer comprising a transcoding device or a decoding device according to the invention.
The advantages of the digital signal processing apparatus, the digital photographic apparatus, the computer, the photocopier and the digital printer are similar to those of the methods which they implement or the devices which they include.
An information storage means, which can be read by a computer or a microprocessor, integrated or not into it, possibly removable, stores a program implementing the coding and/or transcoding and/or decoding method.
In this way, the present invention concerns a computer program product loadable into a computer or stored on a computer usable medium comprising software code portions for performing the steps of the coding and/or transcoding and/or decoding method when it runs on a computer.
Other particularities and advantages of the invention will also emerge from the following description of a preferred embodiment of the invention.